1. Field of the Invention
The present invention broadly relates to a multistage fixed catalytic bed hydroprocessing reactor with separate catalyst addition and withdrawal systems for an upper or top stage having an essentially fixed catalyst bed.
More particularly, the present invention relates to a method and apparatus for hydroprocessing where two or more distinct, fixed catalytic beds are employed in a single onstream reactor for any hydroprocessing application, and where in the upper or top essentially fixed catalyst bed, catalyst particles are added and withdrawn as desired while the single onstream reactor is hydroprocessing a hydrocarbon feed stream in a downflow fashion. The present invention provides a method and a reactor which combines the advantages of a moving-bed reactor with the advantages of a fixed bed reactor within a single reactor. The upper or top essentially fixed catalyst bed functions as a guard catalyst bed for preferably removing organometallic matter in order that one or more subsequent fixed catalyst beds in communication with the upper or top essentially fixed catalyst bed (and in the same single onstream reactor containing the upper or top essentially fixed catalyst bed) may more efficiently perform their respective functions and have a longer catalyst life. The present invention also relates to a method for retrofitting an existing fixed bed reactor to produce a single reactor having the advantages of the present invention.
2. Description of the Prior Art
Hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well known method of catalytically treating such heavy hydrocarbons to increase their commercial value. "Heavy" hydrocarbon liquid streams, and particularly reduced crude oils, petroleum residua, tar sand bitumen, shale oil or liquified coal or reclaimed oil, generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organometallic compounds which tend to deactivate catalyst particles during contact by the feed stream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the range of 212.degree. F. to 1200.degree. F. (100.degree. to 650.degree. C.) at pressures of from 20 to 300 atmospheres. Generally such hydroprocessing is in the presence of catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other metallic element particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurization, hydrodenitrification, hydrocracking etc., of heavy oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one more promoter(s) or catalytically active metal(s) (or compound(s)) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application. Because these reactions must be carried out by contact of a hydrogen-containing gas with the hydrocarbon feed stream at elevated temperatures and pressures, the major costs of such processing are essentially investment in vessels and associated furnaces, heat exchangers, pumps, piping and valves capable of such service and the replacement cost of catalyst contaminated in such service. Commercial hydroprocessing of relatively low cost feed stocks such as reduced crude oils containing pollutant compounds, requires a flow rate on the order of a few thousand up to one hundred thousand barrels per day, with concurrent flow of hydrogen at up to 10,000 standard cubic feet per barrel of the liquid feed. Vessels capable of containing such a reaction process are accordingly cost-intensive both due to the need to contain and withstand corrosion and metal embrittlement by the hydrogen and sulfur compounds, while carrying out the desired reactions, such as demetallation, denitrification, desulfurization, and cracking at elevated pressure and temperatures. Pumps, piping and valves for handling fluid streams containing hydrogen at such pressures and temperatures are also costly, because at such pressures seals must remain hydrogen impervious over extended service periods of many months. It is also cost-intensive to insure that all additional reactor vessels (e.g. fixed bed reactors, etc.) are obtained in order to catalytically process hydrocarbon feed streams from an initial reactor vessel where certain catalytic hydroprocessing (e.g. hydrodemetallation) is/are to be performed to one or more other reactor vessel(s) where additional catalytic hydroprocessing (e.g. hydrodenitrification) is/are to be performed.
Further, hydroprocessing catalyst for such one or more reactor vessel(s), which typically contains catalytically active metals such as titanium, cobalt, nickel, tungsten, molybdenum, etc., may involve a catalyst inventory of 500,000 pounds or more at a cost of $2 to $4/lb. Accordingly, for economic feasibility in commercial operations, the process must handle high flow rates and the one or more reactor vessel(s) should be filled with as much catalyst inventory as possible to maximize catalyst activity and run length. Additionally, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams of varying amounts of contaminants such as sulfur, nitrogen, metals and/or organic-metallic compounds, such as those found in a wide variety of the more plentiful (and hence cheaper) reduced crude oils, residua, or liquified coal, tar sand bitumen or shale oils, as well as used oils, and the like.
The following three acceptable reactor technologies are currently available to the industry for hydrogen upgrading of "heavy" hydrocarbon liquid streams: (i) fixed bed reactor systems; (ii) ebullated or expanded type reactor systems which are capable of onstream catalyst replacement and are presently known to industry under the trademarks H-Oil.sup.R and LC-Fining.sup.R ; and (iii) the substantially packed-bed type reactor system having an onstream catalyst replacement system, as more particularly described in U.S. Pat. No. 5,076,908 to Stangeland et al, having a common assignee with the current inventions and discoveries.
A fixed bed reactor system may be defined as a reactor system having one or more reaction zone(s) of stationary catalyst, through which feed streams of liquid hydrocarbon and hydrogen flow downwardly and concurrently with respect to each other.
An ebullated or expanded bed reactor system may be defined as a reactor system having an upflow type single reaction zone reactor containing catalyst in random motion in an expanded catalytic bed state, typically expanded from 10% by volume to about 35% or more by volume above a "slumped" catalyst bed condition (e.g. non-expanded or non-ebullated state).
As particularly described in U.S. Pat. No. 5,076,908 to Stangeland et al, the substantially packed-bed type reactor system is an upflow type reactor system including multiple reaction zones of packed catalyst particles having little or no movement during normal operating conditions of no catalyst addition or withdrawal. In the substantially packed-bed type reactor system of Stangeland et al., when catalyst is withdrawn from the reactor during normal catalyst replacement, the catalyst flows in a downwardly direction under essentially plug flow or in an essentially plug flow fashion, with a minimum of mixing with catalyst in layers which are adjacent either above or below the catalyst layer under observation.
Of the three acceptable reactor systems, most hydroconversion reactor systems presently in operation on a worldwide basis are fixed bed reactors wherein a liquid hydrocarbon feed and a hydrogen stream flow concurrently through the catalyst beds in a downward flow path. While these fixed bed downflow type processes assure maximum density or volume of catalyst within a reaction zone without expansion of the bed, they are limited by the tendency of the catalyst to form local deposits of feed metals and other contaminates, particularly in the top catalyst bed (i.e. first reaction zone), affecting distribution and reaction rates. As a reactor average temperatures are progressively increased to maintain processing objectives under conditions of increasing local metal deposits, catalyst deactivation due to carbon deposition accelerates. When processing objectives can no longer be maintained due to catalyst deactivation (i.e. normally recognized as "End of Run" conditions), the reactor system must be taken offstream for catalyst regeneration or replacement.
It is known to use a series of individual reactor vessels stacked one above the other, with fluid flow either cocurrent or counterflow to catalyst. In such a process, catalyst moves by gravity from the upper vessel to a lower vessel by periodically shutting off, or closing, valves between the individual vessels. In a counterflow system, this permits removal of catalyst from the lowermost or first stage vessel, where the most contaminated, or raw, feed stock, originally contacts the catalyst. In this way, most of the major contaminating components in the hydrocarbon stream are removed before the hydrocarbon material reaches major conversion steps of the process performed in higher vessels of the stacked series. Thus, most of the deactivating components of the feed stream are removed before it reaches the least contaminated catalyst added to the topmost vessel. However, such systems require valves suitable for closing off catalyst flow against catalyst trapped in the line. Hence, valve life is relatively short and down-time for replacement or repair of the valves is relatively costly. Also, such series of individual reactor vessels are costly since each respective reactor vessel must be purchased separately.
It is also known that contaminants (e.g. metals, subterranean particulates, and organometallic components) are frequently removed from hydrotreater feeds, particularly those boiling above 800E F, by a layer of guard bed catalyst, added atop one or more layers of hydrotreating catalyst. The guard bed catalyst may be relatively inert, removing contaminant particles by trapping them. Alternatively, the guard material may have catalytic activity, removing contaminants such as organometallic feed components by causing them to deposit on the guard catalyst surfaces. Since guard bed catalysts are designed specifically to handle the contaminants, they help to prolong the life of the hydrotreating catalyst and require fewer total catalyst changeouts. However, even changing out the guard bed catalyst in a convention reactor requires a complete reactor system shutdown.
The following prior art does not disclose or suggest two or more distinct, fixed bed catalyst in a single onstream reactor, with the upper or top catalyst bed functioning as a guard bed catalyst that may be replaced without requiring a shutdown:
Jacquin et al. U.S. Pat. No. 4,312,741, is directed toward a method of on-stream catalyst replacement in a hydroprocessing system by controlling the feed of hydrogen gas at one or more levels. Catalyst, as an ebullated bed counterflows through the reactor but is slowed at each of several levels by horizontally constricted areas which increase the hydrogen and hydrocarbon flow rates to sufficiently locally slow downward flow of catalyst. While local recycling thus occurs at each such stage, rapid through-flow of fresh catalyst, with resultant mixing with deactivated or contaminated catalyst, is suppressed. The ebullating bed aids simple gravity withdrawal of catalyst from the vessel. Improvement of the disclosed system over multiple vessels with valves between stages is suggested to avoid the risk of rapid wear and deterioration of valve seals by catalyst abrasion.
Kodera et al. U.S. Pat. No. 3,716,478, discloses low linear velocity of a mixed feed of liquid and H.sub.2 gas to avoid expansion (or contraction) of catalyst bed. By low linear velocity of fluid upflow, gas bubbles are controlled by flow through the packed bed, but the bed is fluidized by forming the bottom with a small cross-sectional area adjacent the withdrawal tube. This assists discharge of catalyst without backmixing of contaminated catalyst with fresh catalyst at the top of the single vessel. The range of the bed level in the vessel is from 0.9 to 1.1 of the allowable bed volume ("10%") due to fluid flow through the bed. A particular limitation of the system is that flow of the fluids undergoing catalytic reaction is restricted to a rate that will not exceed such limits, but must be adequate to ebullate the bed adjacent the catalyst withdrawal tube. Alternatively, injection of auxiliary fluid from a slidable pipe section is required. The patentees particularly specify that the diameter of the lower end of the vessel is smaller to increase turbulence and ebullation of catalyst adjacent the inlet to the catalyst withdrawal line. Fluidization of catalyst is accordingly indicated to be essential to the process. However the disclosed gas flow rates are well below commercial flow rates and there is no suggestion of temperatures or pressures used in the tests or the size, density or shape of the catalyst.
Bischoff et al, U.S. Pat. No. 4,571,326, is directed to apparatus for withdrawing catalyst through the center of a catalyst bed counterflowing to a liquid hydrocarbon and gas feed stream. The system is particularly directed to arrangements for assuring uniform distribution of hydrogen gas with the liquid feed across the cross-sectional area of the bed. Such uniform distribution appears to be created because the bed is ebullating under the disclosed conditions of flow. Accordingly, considerable reactor space is used to initially mix the gas and hydrocarbon liquid feeds in the lower end of the vessel before flowing to other bottom feed distributors. The feeds are further mixed at a higher level by such distributor means in the form of "Sulzer Plates" or a "honeycomb" of hexagonal tubes beneath a truncated, conical, or pyramidal-shaped funnel screen. The arrangement may include an open ramp area parallel to the underside of the screen between the tube or plate ends. Further, to maintain gas distribution along the length of the catalyst bed, quench gas is supplied through upflowing jets in star-shaped or annular headers extending across middle portions of the vessel. The arrangement for withdrawal of spent catalyst requires ebullation of at least the lower portion of the bed. As noted above, added vessel space for uniform mixing of hydrogen and feed before introducing the fluids into an ebullated bed, as well as an ebullating bed, increases the required size of the hydroprocessing vessel, increases catalyst attrition, increases catalyst bed mixing and substantially increases initial, and continuing operating costs of the system.
Bischoff et al. U.S. Pat. No. 4,639,354, more fully describes a method of hydroprocessing, similar to U.S. Pat. No. 4,571,326, wherein similar apparatus obtains uniform ebullation through the vertical height of a catalyst bed, including a quench gas step.
Meaux U.S. Pat. No. 3,336,217, is particularly directed to a catalyst withdrawal method from an ebullating bed reactor. In the system, catalyst accumulating at the bottom of a vessel and supported on a flat bubble-tray may be withdrawn through an inverted J-tube having a particular ratio of the volume of the short leg of the J-tube to the longer leg. The diameter of the J-tube is suited only to flow of catalyst from a body of catalyst ebullated by the upflowing hydrocarbon feed and gas.
U.S. Pat. Nos. 4,444,653 and 4,392,943, both to Euzen, et al., disclose removal systems for catalyst replacement in an ebullating bed. In these patents, the fluid charge including hydrocarbon containing gas is introduced by various arrangements of downwardly directed jets acting laterally against or directly onto the conical upper surface of the bed support screen or screens. Alternatively, the feed is introduced through a conical screen after passing through a distributor arrangement of tortuous paths or a multiplicity of separate tubes to mix the gas and liquid feed over the conical screen. Such arrangements use a considerable volume of the pressure vessel to assure such mixing.
U.S. Pat. Nos. 3,730,880 and 3,880,569, both to Van der Toorn, et al., disclose a series of catalytic reactors wherein catalyst moves downwardly by gravity from vessel to vessel through check valves. As noted above, such valves require opening and closing to regulate the rate of flow, or to start and stop catalyst transfer, with catalyst in the valve flow path. Feed of process fluids is either cocurrent or countercurrent through the catalyst bed.
Van ZijllLanghaut et al. U.S. Pat. No. 4,259,294, is directed to a system for on-stream catalyst replacement by entrainment of the catalyst in oil pumped as a slurry either to withdraw catalyst from or to supply fresh catalyst to, a reactor vessel. Reacting feed is suggested to be either cocurrent or countercurrent with catalyst flow through the reactor. Valves capable of closing with catalyst in the line, or after back-flow of slurry oil, are required to seal off the catalyst containing vessel at operating temperatures and pressures from the receiving reactor vessel, or isolate the catalyst receiving lock hopper from the withdrawal section of the vessel.
Carson U.S. Pat. No. 3,470,900, and Sikama U.S. Pat. No. 4,167,474, respectively illustrate multiple single bed reactors and multi-bed reactors in which catalyst is replaced either continuously or periodically. The feed and catalyst flow cocurrently and/or radially. Catalyst is regenerated and returned to the reactor, or disposed of. No catalyst withdrawal system is disclosed apart from either the configuration of the internal bed support or the shape of the vessel bottom to assist gravity discharge of catalyst.
U.S. Patent No. 3,966,420 to Pegels et al, describes a desulfurization reactor containing at least one tray and catalyst support for one or more catalyst beds, in which the conically shaped support is permeable to fluid and impermeable to catalyst particles. Catalyst may flow through an aperture in the support. A tray located beneath the support is also permeable to fluid and impermeable to catalyst particles. The disclosure allows for more than one tray and support assembly. The tray prevents catalyst particles during loading and unloading from plugging the underside of the support. The acute angle formed by the descriptive line of the conical support surface and the axis of the reactor is taught to be preferable between 35E and 45E. U.S. Pat. No. 4,502,946 to Pronk describes a process for the complete replacement of catalyst from a reactor vessel. The catalyst is removed through a valve which is preferably a rotary valve.
U.S. Pat. No. 4,590,045 to van der Wal, et al. describes a process for preventing catalyst fines from plugging the screen through which products pass within the reactor. U.S. Pat. No. 5,270,018 to Scheuerman describes an inverted J-tube for withdrawing spent catalyst from the bottom of a catalyst bed of a reactor vessel. To remove catalyst from the non-fluidized catalyst bed, the catalyst around the inlet of the J-tube is fluidized sufficiently to initiate and maintain direct transport of catalyst from the reactor vessel.
The method and reactor disclosed by Stangeland et al in U.S. Pat. No. 5,076,908, as well as the method(s) and reactor(s) taught by the above-identified prior art patents relating to U.S. Pat. No. 5,076,908 to Stangeland et al, all suggest that additional reactor vessels are needed to further process hydrocarbon products produced by a reactor vessel containing a single catalyst bed and/or if a single reactor contains two or more fixed catalyst beds, then the single reactor would have to be regularly shut down to replace the fixed catalyst beds. If two or more catalyst beds are needed and they are not to be mixed together, then two or more separate reactors placed in series are required. Separate reactors for separate purposes are expensive. Therefore, what is needed and what has been invented is a method and a reactor that is capable of containing two or more separate and distinct fixed catalyst beds, wherein the upper or top essentially fixed catalyst bed functions as a demetallation guard catalyst bed to remove contaminants (e.g. organometallic components) from a hydrocarbon feed stream before the hydrocarbon feed stream is further hydroprocessed through at least one additional fixed catalyst bed.